The most familiar role of DNA is as a code that specifies the structure of a protein, the role to which the word "gene" was first applied in regard to DNA. The diverse functions of life are brought about by the chemical properties of the 20 different amino acids (listed in Table 4-2); their number and order determine the interactions of a protein with other molecules. These properties depend on how it is folded, chemically modified, and combined with other molecules (including other proteins). Many proteins function as complexes of several polypeptides, each coded by region is of DNA that may or may not be located close to each other on the same chromosome. This simple logic accounts for the diversity of function observed in the biosphere.
Amino acid specification is based on a three-nucleotide genetic code, as listed in Table 4-3. Because there are 20 amino acids but only four nucleotides, at least three nucleotides are required to specify 20 different amino acids. However, because there are 43 = 64 possible different three-nucleotide codes and the code is redundant, most amino acids are specified by more than one triplet, in ways that probably reveal the evolutionary history of the coding system (see below). Within a protein-coding segment of DNA, each successive three-nucleotide stretch comprises a codon, whose sequence specifies a single amino acid in the coded protein. Most amino acids are specified by more than one codon (code redundancy), and three codons specify
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